I usually match mine to my neck. My face is considerably lighter/more pale. It all evens out once the foundation is on. I have found that certain formulations oxidize more than others. I've just tried to stay away from those.
Also anything with an SPF in it - especially liquid dries lighter for some reason. The heavier coverages - like the studio fix, the stick, the mineralize satinfinish, the studio tech oxidizes and turns dark. I typically don't wear any of those for that reason. They turn me red once they oxidize and whatever other products (like blush, bronzer, etc.) I put on top red too. Sometimes switching to NC from NW will help though. I can wear the stick in NC but in NW it goes red and wacky.
You also have to take your skin type into consideration too. If you are oily, your makeup is more likely to oxidize. If you are dry, a powder foundation alone may look ashy. Liquid always works best for me. The creamy products are too heavy and greasy. They go dark and oxidize and the powders make me look like a ghost.